Ink-jet printers have rapidly gained in popularity as a means for generating high quality gray scale and color images from computer sources. Because of the large drop size produced by most ink-jet printers, the color and gray scale images they produce are limited to less than 100 color or gray tones. Although this is adequate for certain applications such as bar graphs and pie charts, it is unquestionably inadequate for making accurate representations of real-world colors and does not approach a photographic appearance.
An ink-jet device makes color images by physically mixing ink on the print medium to obtain a desired color. Inks for a color ink jet printer are generally selected from one or more of black, magenta, cyan, and yellow.
Presently known devices operating at 300 dpi, wherein each of the dots are sized to fill a 0.08.times.0.08 mm.sup.2 picture element (pixel) for example, are limited in the color density and shade, and thus realism that they can provide as a result of this limitation. However, as the number of ink droplets deposited per pixel is increased by making the droplets smaller, it is possible to apply a sufficient number of droplets of ink within a pixel to obtain natural coloring. With respect to gray scale printing, it similarly allows a greater number of shade levels. It is known that a distribution of zero to thirty droplets per 0.08.times.0.08 mm.sup.2 pixel, per color creates hundreds of different density levels per color discernable by the human eye. Thus, it would be desirable to provide a group of nozzles capable of providing thirty or more droplets per pixel.
In addition to the appropriate control software and inks, the above described color image production using ink-jet technology requires an ink nozzle capable of consistently dispensing the very small droplets of ink. Any dispersion of ink caused by imperfections in the ink nozzle has disastrous consequences for print quality, especially when the spray pattern of four nozzles must be coordinated.
In order to obtain photographic quality gray scale and color images using an ink-jet device, an ink nozzle with an orifice approximately 15 microns or less in diameter is required. It is known in the art to fabricate ink nozzles from glass, but because of the difficulty in manufacturing a nozzle from a glass tube having such a small orifice size, it is desirable to provide a method of consistently providing nozzles of the precise dimensions needed.
One prior art ink nozzle is illustrated in U.S. Pat. No. 3,393,988 to Blumenthal, wherein a nozzle having an orifice 0.003 to 0.0004 inches is formed by heating the lower end of a vertically oriented, low melting point, glass tube with a flame burner until it melts into a tear-drop shape under the influence of gravity, thereby forming a converging inner passage that is abruptly tapered (60.degree. to 90.degree. with respect to the central axis of the passage). Glass at the end of the tube is then removed to establish an abruptly converging passageway with a central orifice which is subsequently flame polished to provide smooth surfaces.
It should be noted that Blumenthal's requirement that the tube be oriented vertically, due to the technique's reliance on the force of gravity, is a severe manufacturing limitation. It should also be noted that Blumenthal specifically teaches away from a gently tapered converging portion leading to the orifice. Were such an abruptly tapered end as shown in Blumenthal be ground in an attempt to provide an orifice ten times smaller, with the perfection and symmetry required by photographic quality color printing, the results would be uncertain. Furthermore, even if a 15 micron or less diameter orifice were to be obtained, the flame polishing step of Blumenthal would produce an unacceptable change in the diameter of the orifice with respect to the requirements for the above described color printing application.
The drawing or pulling method of making a converging passage, specifically rejected by Blumenthal, is described in U.S. Pat. Nos. 3,985,535 and 4,111,677. As Blumenthal indicates, drawing a glass tube causes a reduction in passage diameter gradually over such an extended distance that it causes fluid flow problems. In order to draw a glass tube, a relatively large portion of the tube must be made molten and glass in its molten state is very hard to dimension with accuracy.
Additionally, pulling a heated glass tube to cause narrowing of a central passage causes a concomitant reduction in wall thickness. The resulting drawn portion of the glass tube is therefore extremely fragile even at the diameters taught by Blumenthal, and is extraordinarily so in a tube one tenth the size. The susceptibility of the drawn glass nozzle to material failure is exacerbated by the forces applied by a mechanically stimulated piezo-electric crystal used in some ink-jet devices to assist in uniform droplet formation. Reinforcement of the fragile drawn tube is demonstrated in a fluid dispensing device, the 9103557E173E manufactured by Siemens-Elma AB of Sweden, which provides a metal sheath over the tube, except in the area of the orifice where the tube is uncovered.
Therefore, in addition to the other above-recited features lacking in the prior art, it would be desirable to provide a tube having an inner wall leading to an orifice with a less extensive taper than a drawn tube, yet more taper than the Blumenthal tube, with an orifice in the fifteen micron or less range. It would further be desirable to form such a tube without weakening it so that it is unmanageably fragile or so that it requires reinforcement.